1. Field of the Invention
This invention relates to a projection lens system, and more specifically to a projection lens system which is particularly suitable for a motion picture projector. The angle of view of the lens system is 21.degree. and the F-number thereof is F:1.3.
2. Description of the Prior Art
In a motion picture projector, it is desirable to use a projection lens which has a comparatively large angle of view and high resolving power. In accordance with the development of the motion picture film of 8mm, the effective area of one frame of the film has been increased by as much as 50 percent of the conventional one. In order to make good use of the improved 8 mm film, it is often desired to improve the projection lens. Particularly, are desired projection lenses which have high resolving power and a wide angle of field.
More concretely, the projection lens system to be employed in the 8mm motion picture projector is desired to satisfy the following conditions.
First, the brightness of the lens system is desired to be as high as F:1.0 to 1.3 in F-number in order to enhance the efficiency of the light source which may be a halogen lamp or the like. Second, the angle of view is desired to be not less than 20.degree. so that a large image may be obtained in a small projection room. Third, the lens system is desired to be free from off-axis comatic flares and have good flatness of image surface in order to obtain a sharp image of high contrast wherein the marginal portion of the image is sharp even when the aperture is fully opened. Fourth, the chromatic aberration should be as small as possible. Fifth, the aperture efficiency is preferred to be large enough to illuminate the projected plane. Sixth, the lens should preferably be easily manufactured at a low cost.
As the conventional projection lens system there has been well known and widely used an Ernostar type lens system consisting of four lens components constituted of four lenses. This type of the conventional projection lens is characterized in that the brightness can be made as high as F:1.3 to 1.4 in F-number, although the angle of view is not so large. However, in general in this type of lens, it is difficult to make the Petzval sum not more than 0.5, and the flatness of the image surface is markedly degraded when the angle of view is made as large as 20.degree.. Further, the spherical aberration will increase if the brightness is made as high as F:1.3. Therefore, the quality of the image obtained by this type of lens is unsatisfactory when it is employed in a middle or high class projector using a halogen lamp.
As the conventional projection lens system of high performance there has been known and used a so-called Hi-Fi lens. As a Hi-Fi lens of the brightness of F:1.3 to 1.4 there is known a Gauss type lens system consisting of four lens components constituted of six lenses. This type of lens is characterized in that the spherical aberration is well corrected when the brightness if F:1.3 to 1.4, and in the range of the angle of view of about 20.degree. the off-axis comatic flares are well reduced and the Petzval sum is controlled to be 0.2 to 0.3 and the astigmatism can easily be corrected. However, this type of lens is difficult to manufacture since the radius of curvature of the third and sixth surfaces must be made about 0.4f and the radius of curvature of the fifth surface must be made about 0.3f (f is the effective focal length of the lens system) in order to make good correction for aberrations possible. Further, since the second and third lens components must be cemented lens components, the cost of manufacture becomes inevitably high.